1. Field of Invention
The present disclosure relates to recovery of base metal values, such as nickel, cobalt, copper and the like, from oxide type materials.
2. Description of Related Art
There are several methods available to those skilled in the art for the extraction of nickel and other base metals from oxide ores, and especially from laterites. These conventional methods generally have several disadvantages that render producing nickel from laterites a difficult task.
The choice of a hydrometallurgical route for nickel laterites is highly dependent on ore characteristics because no conventional process can be generally applied thoroughly. Parameters such as flexibility, high recovery, and savings of energy, reagents and water are not only desired, but are essential to a viable hydrometallurgical plant. There are several leaching options available for nickel laterites, such as the Caron process (roasting/reducing/ammonia-based leaching) or the pressure acid leaching (PAL or HPAL—high pressure acid leaching), but these leaching options are generally associated with high operational and capital costs. There are other options being developed, trying to reduce those costs, such as atmospheric acid leaching in agitated tanks or heap leaching (McDonald and Whittington, 2007; Whittington, McDonald, Johnson and Muir, 2002).
Even though there are commercially available hydrometallurgical options for nickel laterites and plenty of development in the field with good technical background, these options are still costly. High capital and economical costs are related to the number and complexity of unit operations that are currently needed for nickel extraction and their complexity. Researchers have been struggling to find solutions for issues such as high acid consumption, impurities extraction, solid/liquid separation, among others. Hydrometallurgical purification of nickel often deals with high volumes and low concentration of valuable metals, thus considerably increasing overall operational costs.
High acid consumption is one of the main components of operational costs of laterite leaching. Nickel and the other base metals are usually bonded in ferruginous ores, as in limonites, or in saprolite matrixes, both being rich in magnesium. Accordingly, in order to effectively leach those elements, iron and magnesium need to be leached, both iron and magnesium being available in high amounts and thus increasing overall acid consumption. That is the main issue with conventional atmospheric or heap leaching operations, as seen throughout the available literature, for example in patent applications WO/2010/000029, WO/2009/146518, WO/2009/018619, EPI 790739 and many others. An excess of acid is needed to achieve high extractions of payable metals. It is known for those skilled in the art that high pressure acid leaching (PAL or HPAL) can deal with the high ferruginous ores, as all iron is hydrolyzed, but magnesium remains an issue.
Document WO 2010000029 (BHP Billiton SSM) teaches a process for the recovery of nickel and cobalt from a nickeliferous oxidic ore by heap leaching and/or atmospheric agitation leaching, the process generally including mixing a sulfur containing reductant selected from reductants that do not include copper into a nickeliferous oxidic ore, leaching the reductant/ore mixture with an acidic leach reagent to produce a pregnant leach solution including nickel, cobalt, iron substantially in a ferrous form and other acid soluble impurities, and recovering the nickel and cobalt from the pregnant leach solution.
WO 2009146518—(VALE S.A.) describes a process of recovering nickel and cobalt and regenerating the main raw materials, the process including granulometric separation, leaching, neutralization, mixed hydroxide precipitate (MHP) production in only one stage and the pressure crystallization of magnesium sulphite. The process proposes a way to recover nickel and cobalt from laterite ores through atmospheric and heap leaching with staged addition of ore—by size separation—and H2SO4, decreasing the nickel losses, simplifying the neutralization circuit and producing a more purified MHP. The present process route is employed for nickel extraction, including the one from high magnesium containing lateritic ores.
WO 2009 018619 (BHP Billiton SSM) describes an atmospheric leach process in the recovery of nickel and cobalt from lateritic ores, the process including providing limonitic and saprolitic ore fractions of a laterite ore, separately slurrying the limonitic and saprolitic ore fractions to produce a limonitic ore slurry and a saprolitic ore slurry, separating any limonitic type minerals from the saprolitic ore slurry to produce a saprolitic feed slurry, milling or wet grinding the saprolitic feed slurry, leaching the limonitic ore slurry with concentrated sulfuric acid in a primary leach step, introducing the saprolitic feed slurry to the leach process in a secondary leach step by combining the saprolitic feed slurry with the leached limonite slurry following substantial completion of the primary leach step, and releasing sulfuric acid to assist in leaching the saprolite feed slurry, wherein the saprolitic feed slurry is substantially free of all limonitic type minerals before it is introduced to the leach process.
EP 1790739 (Companhia Vale do Rio Doce) teaches a process for extraction of nickel, cobalt, and other metals from laterite ores by heap leaching, and of the product obtained as well, characterized by the fact that it is comprised of crushing, agglomeration, stacking and heap leaching stages, with this last stage being a continuous, countercurrent, heap leaching system with two or more stages, comprised of two phases, one of which is composed of the ore, or solute, and the other is composed of the leaching solution, or solvent, which are supplied at opposite ends of the series of stages and flow in opposite directions. Upon cessation of leaching in the last stage, its solute is removed and a new stage is introduced at the first position, formed by new ore to be leached by the solvent solution, which is introduced from the last stage, percolating or flowing through all the previous stages until it reaches the first stage, being separated if loaded with target metals.
Another issue with acid leaching of oxide base metals ores is neutralization and solid-liquid separation. A neutralizing agent, such as, but not limited to, lime, limestone or magnesia, is needed to increase solution pH and hydrolyze some impurities from solution. This operation produces hydroxides, as ferric hydroxides, that make solid-liquid separation very onerous. Rheology is often a problem too. To avoid that problem, high dilution of the solution is needed, and higher volumes of poorer solution are needed to be purified.
Effluent treatment could also be an issue, as magnesium levels can be prohibitive. There are several methods for removing magnesium from solution, but all come with a high cost. Solid residue is also not very stable and needs large tailings ponds.
One patent application, WO/2009/026694, from Berni et al, attempts to address the above-discussed issues by contacting HCl gas and oxide ore. This patent application uses the fact that iron, aluminum and magnesium chlorides can be selectively decomposed from payable metals, then recovering HCl and producing a much cleaner solution to treat and that is free of iron, magnesium, manganese or aluminum. This technique also produces a stable solid residue and in smaller quantity.
The major hurdle on HCl usage for base metals extraction has normally been focused on the need to use highly corrosion resistant materials and to control hydrogen chloride gas emissions.
Gybson and Rice (1997) showed the advantages of hydrochloric acid usage for nickel laterite extraction. There is substantial literature examining the use of hydrochloric acid and several new processes proposed in recent years based upon novel chemistry only achieved in strong chloride liquors.